The invention relates to a sensor with a plurality of sensor elements, each of which includes a radiation-sensitive conversion element which generates an electric signal in dependence on the incident radiation, and also with means for amplifying the electric signal in each sensor element and a read-out switching element in each sensor element which is connected to a read-out line in order to read out the electric signal. The invention also relates to a method of operating such a sensor as well as to an X-ray examination apparatus which includes an X-ray source for emitting an X-ray beam for irradiating an object so as to form an X-ray image, as well as a detector for generating an electric image signal from said X-ray image.
Large-surface X-ray detectors are customarily used for X-ray examination applications, notably in the medical field; such detectors consist of a plurality of sensor elements. The sensor elements (pixels) as a rule are arranged in rows and columns in a sensor matrix. Preferably, use is made of the so-called flat dynamic X-ray detectors (FDXD). Such detectors are seen as universal detector components that can be used in a wide variety of X-ray apparatus.
In contemporary FDXD embodiments, the individual sensor elements (matrix cells) comprise a radiation-sensitive conversion element, having an intrinsic storage capacity, and a switching element for reading out the signal present on the conversion element or the storage capacitance after the irradiation. The FDXD preferably utilize conversion elements in the form of photodiodes of amorphous silicon and scintillator elements connected thereto, or alternatively photoconductors, for the direct conversion of the X-rays into electric charges. In other types of sensors for other radiation, of course, other conversion elements can also be used.
Diode switches or transistors, notably TFTs (thin film transistors) of amorphous silicon are preferably used as read-out switching elements. In order to read out the signal, present as a collected charge on the conversion element or the intrinsic storage capacitance thereof, the read-out switching elements are activated and the collected charge is conducted to the relevant read-out line. From there it flows to a charge-sensitive amplifier (CSA). Subsequently, corresponding electronic information is applied to a multiplexer which conducts this information to a data acquisition unit for display on a display device in the form of a monitor.
When such detectors are used, notably in the medical analysis practice, it is desirable to reduce the radiation dose so as to limit the dose whereto the patient is exposed; consequently, only a very small amount of radiation is incident on the individual sensor elements. As a result, the electric signal in the individual sensor elements is also very small. Therefore, the aim is to realize sensors or X-ray detectors having an as high as possible signal-to-noise ratio.
A particularly high signal-to-noise ratio and detection of small doses, of course, is also desirable for other radiation-sensitive sensors. In order to improve the signal-to-noise ratio, the signal can in principle be amplified already in the individual matrix cell of the detector.
U.S. Pat. No. 5,825,033 discloses a semiconductor detector for gamma rays in which the charge generated in each pixel in the detector material is stored in an integration capacitor of a capacitive feedback amplifier. This integration takes place for all pixels simultaneously. In a so-called Correlated Double Sample-and-Hold circuit (CDSH) the noise induced by the resetting of the integration capacitor is eliminated. Subsequent to the CDSH, the individual pixels are connected to a respective unity gain buffer which is connected to a read-out line common to each column. The read-out lines are then combined by appropriate multiplexers. The sensor in this case consists of a matrix with 48xc3x9748 individual pixels.
For amplifier circuits for enhancing the signal-to-noise ratio, the signal amplification and the noise are customarily the essential characteristics considered for evaluation. For practical operation there is a further criterion in the form of the stability of the transfer function. For example, when the signal amplification or an offset value of the amplifier fluctuates in time, offset and gain artefacts occur in the imaging detector system; such artefacts can only be corrected partly and with great effort only. Such fluctuations may be caused by changes of the temperature or other operating conditions as well as be due to aging, radiation damage and/or trapping effects in semiconductors.
The threshold voltage and the transconductance are liable to change significantly in time, notably in the frequently used thin film transistors (TFTs) of amorphous silicon, which can also be used notably for the manufacture of integrated amplifier circuits in a matrix cell; this may degrade the stability of the transfer function.
Therefore, it is an object of the present invention to provide a sensor and a method of operating the sensor wherein a high stability of the transfer function and an attractive signal-to-noise ratio are ensured by a comparatively simple and economical construction.
This object is achieved by means of a sensor which is characterized in that the means for amplifying include a respective source follower transistor whose gate is connected to the conversion element, whose source is connected an active load and to one side of a sampling capacitor, the other side of the sampling capacitor being connected to the read-out line via the read-out switching element, and that a respective reset element is connected to the conversion element in order to reset the conversion element to an initial state.
The active load ideally constitutes a current source which impresses a constant channel current on the source follower transistor. The threshold voltage of the source follower transistor is thus stabilized; this threshold voltage is strongly dependent on the channel current, notably in the case of TFTs of amorphous silicon. As a result of the stable threshold voltage, the condition for correct operation of the source follower transistor with adequate stability of the transfer function is satisfied. Therefore, the source follower transistor has a stable voltage amplification of 1. It is converted into a charge amplification GQ=CS/CP by the sampling capacitor, wherein CP is the capacitance on the conversion element and CS is the capacitance of the sampling capacitor. The capacitance on the conversion element may again be an intrinsic storage capacitance of the conversion element or an additional capacitance.
Preferably, the active load, the read-out switching element and the reset element are also formed by transistors. All components required for the invention can then be integrated directly in the sensor elements while using the thin film technology which is used any way to form the sensor elements; in the context of this technology the transistors can be made of amorphous silicon or polycrystalline silicon. Because of the stable amplification circuit constructed in conformity with the invention, the use of the TFT transistors of amorphous silicon that can be economically manufactured is not a drawback.
A process with vertical integration can now be advantageously used in such a manner that the surface area of the conversion element, or the storage capacitance within a sensor element, is not reduced.
In one embodiment a discharge switching element, preferably in the form of a transistor, for example a TFT of amorphous or polycrystalline silicon, is connected parallel to the sampling capacitor. This discharge switching element can be used for the simultaneous, accelerated discharging of the sampling capacitor during a reset of the conversion element by means of the reset element, so that the sampling capacitor is also reset to an initial state.
The reset element and the discharge switching element may then have a common switching line so that they are always activated simultaneously. However, they may alternatively have separate switching lines, so that the reset element and the discharge switching element can be individually activated, for example for given modes of operation. Preferably, a plurality of sensor elements, for example all sensor elements of a row of the sensor matrix, have a common switching line for the activation of the read-out switching elements. Such sensor elements, connected to a common switching line, can also have common switching lines, or a common switching line for both elements, in order to activate the reset elements or the discharge switching elements.
According to a particularly advantageous method of operating a sensor according to the invention the conversion element and the sampling capacitor are reset to an initial state during a measuring and read-out cycle in each sensor element in a first phase. In a second phase a voltage difference which is representative of the conversion element in the initial state is then adjusted across the sampling capacitor. During a third phase the voltage across the sampling capacitor is sustained during irradiation of the conversion element by means of a radiation source whereas the voltage at the source output of the source follower is forced to change by the change of the signal at the conversion element or of its capacitance. Evidently, in this context the term irradiation by means of the radiation source is to be understood to mean not only direct irradiation by the radiation source, but also indirect irradiation, for example after transmission through an object to be examined. During a fourth phase the voltage difference across the sampling capacitor is adjusted to a value which is representative of the conversion element after the irradiation, the variation of the potential at the side of the sampling capacitor which is connected to the read-out line then being measured as a measure of the radiation incident on the conversion element. Preferably, the variation of the charge at the read-out side of the sampling capacitor is then recorded in a charge-sensitive amplifier (CSA). This means that the amount of charge flowing during the adjustment of the new voltage difference is integrated.
As a result of this switching sequence a so-called xe2x80x9ccorrelated double samplingxe2x80x9d (CDS) method is implemented in the relevant sensor element. This means that during the second phase a first sampling value is detected for the conversion element in the stationary state whereas during the fourth phase ultimately a value is measured across the sampling capacity which corresponds to the conversion element after the irradiation, only the difference between the initial state and the irradiated state being measured during the first sampling because of the bias in the second phase.
This switching process also offers the advantage that the reset operation during the first phase lies outside the time interval in which the conversion element is irradiated and the signal is read out, so that the reset operation has no effect on the measuring result and hence cannot contribute to the noise.
According to a second version of the method of the invention, a dark current is first detected on the conversion element during a first sub-phase of the second phase, the voltage difference across the sampling capacitor being held during a given time interval without irradiation of the conversion element by the radiation source while at the same time the voltage on the source output varies in conformity with the dark current occurring across the conversion element. The dark current can then be attributed essentially to leakage currents on the conversion element. This sub-phase is succeeded by a second sub-phase during which a voltage difference is adjusted across the sampling capacitor, which voltage difference corresponds to a reference state of the conversion element after the detection of the dark current. The second sub-phase is succeeded by a third sub-phase in which the conversion element is reset to its initial state, the voltage difference across the sampling capacitor then being maintained. The execution of the other phases is the same as in the previously described method.
The difference between this method and the previously mentioned mode of operation thus consists in that during the first sampling operation the initial state, that is, the off-load voltage on the conversion element, is not taken as the reference value, but the reference state already contains the integrated dark current. This means that the dark images are already buffered in the individual sensor elements and subtracted from the exposed images. Thus, the transfer and external storage of the dark images is dispensed with. Additionally, the usable dynamic range of the sensor is expanded, since the charges transferred from the individual sensor elements no longer contain a dark current component.
The adjustment of the voltage difference across the sampling capacitor during the second and the fourth phase is performed most easily by activation of the read-out switching element, that is, via the read-out line. In order to sustain the voltage difference during the third phase, or during the dark current measurement, the read-out switching element need only be deactivated.
The resetting of the sampling capacitor in one embodiment of the invention can be realized by activation of the discharge switching element connected parallel to the sampling capacitor, thus enabling accelerated resetting.
The measuring and read-out cycles can be controlled in common for each time a plurality of sensor elements and via common switching lines. That is, after the irradiation the sensor elements in a sensor matrix are successively read out in rows and are reset.
According to a third version of the method of the invention, there is provided a method of operating a sensor having a plurality of sensor elements (10) arranged in rows and columns, each of which includes a radiation-sensitive conversion element (1) which generates an electric signal in dependence on the incident radiation, a reset element (27) which resets the conversion element (1) to an initial state, and a source follower transistor (21) whose source is connected to an active load (23) and to one side of a sampling capacitor (26) whose other side is connected, via a read-out switching element (30), to a read-out line (8) for reading out the electric signal, the method comprising:
resetting the radiation-sensitive element and charging the sampling capacitor of each pixel to a known voltage;
exposing the sensor to radiation, the radiation-sensitive conversion element causing the voltage on the one side of the sampling capacitor to vary, wherein the read-out switching element is open during the exposure, providing an open circuit at the other side of the sampling capacitor, thereby maintaining a constant charge on the sampling capacitor; and
closing the read out switching elements and charging the sampling capacitor for each pixel in a row to the voltage on the one side of the sampling capacitor, the amount of charge required being measured.
By separating the pixel resetting from the array readout phase, this scheme provides sufficient time for the sampling capacitor to reach a steady state which eliminates the pixel offset error charge. Furthermore, the pixel readout time can be increased.
According to a fourth version of the method of the invention, there is provided a method of operating a sensor having a plurality of sensor elements (10) arranged in rows and columns, each of which includes a radiation-sensitive conversion element (1) which generates an electric signal in dependence on the incident radiation, a reset element (27) which resets the conversion element (1) to an initial state, and a source follower transistor (21) whose source is connected to an active load (23) and to one side of a sampling capacitor (26) whose other side is connected, via a read-out switching element (30), to a read-out line (8) for reading out the electric signal, the method comprising:
exposing the sensor to radiation with the read-out switching elements closed, the radiation-sensitive conversion element causing a change in the voltage on the one side of the sampling capacitor and the read out line holding the other side of the sampling capacitor to a constant voltage;
closing the reset elements to and opening the read out switching elements, thereby holding the conversion element to a constant state irrespective of the incident radiation;
closing the read out switching elements of each row in turn and measuring the charge stored on the sampling capacitors for each row in turn.
Since the reset switches remain on during the readout period, the photodiode charge remains constant. Therefore, radiation incident on the detector after the exposure time will not alter the signal being readout through the sampling capacitors so that Frame Transfer operation is possible.
An X-ray examination apparatus according to the invention includes an X-ray source for emitting an X-ray beam for irradiating an object so as to form an X-ray image, as well as a detector for forming an electric image signal from said X-ray image, the X-ray detector being equipped with a sensor according to the invention. Such an X-ray examination apparatus has a particularly attractive signal-to-noise ratio and, therefore, is capable of operating with small doses, so that the radiation load for the object, notably a patient, can be kept small.